gauche: disable on SPARC(64), remove atomics dependency

gauche does not build on sparc64, it fails with:

  No rule to make target 'sparc_mach_dep.lo', needed by 'libgc.la'

On Sparc, it first fails with the usual "membar" assembler issue which
comes from gauche's internal version of libatomic_ops. However, even
passing -DAO_NO_SPARC_V9 like we do for libatomic_ops is not
sufficient, as we fall into the same build problem as on sparc64.

Consequently, this commit disables the gauche package on the sparc and
sparc64 architectures.

Since it uses its own libatomic_ops copy, and libatomic_ops does not
have any dependency on atomic built-ins, we also remove the
BR2_ARCH_HAS_ATOMICS dependency. It was originally added to prevent
gauche from being built on sparc, but this will no longer happen.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>